The Roman number system is based on seven letters, all of which are assigned with specific values. These seven letters, with their Hindu-Arabic number equivalents, are:

 I (1)       V (5)      X (10)      L (50)      C (100)      D (500)      M (1000)

There are a few rules in the Roman number system.

Rule 1: When a letter is repeated, its value is repeated.
Examples : I = 1, II = 2, III = 3          X = 10, XX = 20          CCC = 300

Rule 2: When a letter follows a letter of greater value, its value is added to the greater value.
Examples : VI = 6 (5+1)       XV = 15 (10+5)        LX = 60 (50+10)        DC = 600 (500+100)

To write 70 in roman numerals, we just need to add XX to L (10+10+50). So, 70 will therefore be written as LXX. To write 800, just add CCC to D (100+100+100+500). In this case, 800 is written as DCCC.

Rule 3: When a letter precedes a letter of greater values, its value is subtracted from the greater value.
Examples : IV = 4 (5-1)                       IX = 9 (10-1)                      XL = 40 (50-10)
                   XC = 90 (100-10)             CD = 400 (500-100)

The smaller value I is in front of V, means subtract 1 from 5 to get 4. In the same way, X is in front of the L reduces 50 by 10 to get 40. In like manner, X is in front of C means 100 less 10 and CD denotes 500 less 100.

Note: Generally, the symbols are not repeated more than three times to denote a number. To show the number 40, we would need to write XL (50-10) and not XXXX. (10+10+10+10). As for 4, it is usually written as IV (5-1) and not IIII (1+1+1+1).

Rule 4: A horizontal bar over a letter or letters indicates that the value given to the letter or letters is to be increased one thousand times.
Examples : MCD = 1,400 (1000+ 500-100).
____
MCD = 1,400,000 (1,000 times of 1,400)

Summary

VII = 7          XI = 11          XIV = 14         XVIII = 18         XXII = 22      XXXVII = 37
XLI = 41       LXII = 62       CXIII = 113     CCX = 210      MCM = 1,900

__                              __                               ______    
XICCC = 11,300       CLXX = 150,020         MCMV = 1,905,000  

 __                         __
 M = 1,000,000       XCC = 10,200

Source : “Arithmetic Made Simple – Revised Edition” by A.P. Sperling and Samuel D. Levison. Revised by Robert R Belge, Department of Electrical and Computer Engineering, Syracuse University. Published by Doubleday, a division of Bantam Doubleday Dell Publishing Group, Inc.